1. Field of the Invention
The present invention relates to a method of manufacturing an anchor element for a soil anchor or rock anchor, rock bolt or the like, from a strand of twisted steel wire. The anchor element includes a central wire and outer wires arranged radially relative to the central wire. The anchor element has at least one expanded section obtained by spreading the individual wires. Spacer members are provided for fixing the individual wires in the spread-apart position.
2. Description of the Related Art
For securing excavated cavities in mining, particularly coal mining, it is known to use rock bolts which are usually placed immediately after driving the cavity. Such rock bolts are usually composed of an anchor element of steel, for example, a steel rod, a steel pipe or the like, which is placed in a bore hole and a bonding action between the anchor element and the rock is achieved by filling the bore hole with a hardening material, for example, cement mortar or synthetic resin.
Longer rock bolts of the above-described type cannot be used, or can only be used under great difficulties for longer bore holes, sometimes up to 15 m, because of the limited space available on location within narrow cavities. Accordingly, anchor elements of steel wire strands are frequently also used as rock bolts. Steel wire strands are made from twisted high-strength steel wire; they are flexible within certain limits and can be bent for inserting them within a bore hole. However, steel wire strands have a very smooth surface because of strain hardening during drawing, so that the forces resulting from the bonding action between the strands the hardening material filling out the bore hole are frequently not sufficient for preventing the separation of layers in the rock formation.
In order to improve the bonding action of steel wire strands when used as rock anchors, it is known to produce expanded sections by spreading the individual wires into which the material can penetrate, in order to improve the bonding action. The expanded sections are arranged spaced apart from each other over the length of the rock anchor. The expanded sections are produced in a method of the above-described type by guiding the strand in an axial feeding direction through a rotatably mounted spreading disk which is provided with openings and by mounting spacer elements for fixing the individual wires in the spread-apart position in the areas where the individual wires have been spread apart by the spreading disk, as disclosed by DE 42 03 740 A1. The spacer elements are constructed and arranged in such a way that the radial symmetry of the strands is not impaired in the areas of the expanded sections, i.e., the central wire extends through a central opening of a circular or polygonal spacer element, while the outer wires rest against the outer circumference of the spacer elements.
As a rule, steel wire strands as they are used in this connection are composed of seven steel wires, wherein six outer wires are grouped around a central wire. When the outer wires are bent outwardly as a result of the spreading action for mounting the spacer elements, differences in length occur in the final state between the outer wires and the central wire. In the case of short strands, or when expanded sections are provided only over the short anchoring length of the strand, these length differences can be compensated by longitudinal displacement of the central wire. However, this phenonemon prevents a continuous manufacture of longer strands in a simple continuous process because the central wire will laterally buckle toward the outer wires after a limited number of expansions because of the fact that the length differences add up. It would be possible to sever the central wire at axially spaced-apart locations, or to cut out individual pieces of the central wire at certain locations; however, this would require additional work steps which prevent an economical manufacture.